The majority of the known and commercially available low-pressure fluorescent discharge lamps are so-called compact fluorescent lamps (CFL-s) at present. These lamps are intended to replace incandescent lamps used in a wide field of industry and home applications. Main advantages of these lamps are a low-power consumption and a long lifetime. Disadvantageous is however in CFL-s their relatively high price and large length dimension. Many configurations have been proposed to solve the length dimension problem. Such solutions include the multiple tube arrangements and the coiled tube arrangements.
U.S. Pat. No. 6,064,155 discloses a fluorescent lamp with an outer envelope having an external shape of an incandescent lamp on a standard Edison-type base. The discharge tube is wound in a coil around the longitudinal axis of the lamp and is disposed within the outer envelope. Ballast is also disposed within the outer envelope. In order to place the discharge tube within the outer envelope, the envelope is cut midway at a seam and then resealed after placement of the discharge tube. It is not disclosed and therefore it is not clear from this document how the envelope is cut and resealed so as to form a uniform bulb shape. In case of a glass envelope, the two separated parts have to be welded which causes a thickened circumferential seam area that has a negative impact on the optical characteristics and aesthetic appearance of the lamp. In addition to this, the increased wall thickness in the seal region leads to excessive stresses in the glass wall of the envelope.
Accordingly, there is a need for an electric lamp, in particular a compact fluorescent lamp, with an outer envelope, preferably of a glass material with an improved wall construction in the seam area of the outer envelope that allows the lamp to be manufactured without substantial increase of costs. There is also need for an improved method of production, which is easy to combine with the conventional manufacturing steps and therefore compatible with mass production. It is sought to provide a compact fluorescent lamp configuration, which readily supports different types of discharge tube configurations.